1. Field of the Invention
This invention relates to a method of producing a high pressure fuel injection pipe often used as a fuel supply line for a Diesel internal combustion engine, and more particularly to such a method which comprises the steps of cutting off the hair crack layer portion including the black skin layer of the inner wall surface of a steel blank pipe, and thereafter repetitively subjecting said steel blank pipe to the process of pipe dilation and the annealing process within a non-oxidizing hearth, thereby forming a finished pipe.
2. Description of the Prior Art
In the prior art method of producing such a type of injection pipe, a steel blank pipe whose inner peripheral wall surface has a black skin layer and hair crack layer portion of the order of 0.1 to 0.15 mm in thickness has immediately been repetitively subjected to the process of pipe dilation and the process of annealing within a non-oxidizing environment.
According to this method, however, numberless crack layers which have grown into a black skin layer have remained in the finished pipe of desired final diameter, so that the roughness of the inner peripheral wall surface of the finished pipe has been 8S or higher (see the photograph of FIG. 2B of the accompanying drawings) and the black skin layer has been peeled off by the internal fluid pressure and in addition, the numberless crack layers so grown have further been grown with the use of the pipe, thus resulting in deteriorated resistance to pressure of the pipe which have often led to fractures or breakage of the pipe during the use thereof.